1. Field of the Invention
The present invention relates to a function for optimally controlling a value of a compression parameter when compressing the image data that is photographed by a digital photographic device, and particularly, the present invention relates to the optimum method in the case of mounting a control function of the compression parameter value of such image data in a camera-equipped cellular phone and a camera-equipped PDA or the like and a digital photographic device that is provided with the structure for this method.
2. Description of the Related Art
In recent years, there are many occasions that a camera is incorporated in an electric device such as a personal computer, a PDA, and a cellular phone. These devices also can photograph a still image and a moving image as same as a digital camera that is a conventional specialized photographic machine. However, there is a function to estimate number of remaining images to be shot as a function that is mounted in the digital camera that is the specialized photographic machine but that has not mounted yet in a multi-function cellular phone, PDA, or the like such as a camera-equipped cellular phone.
The remaining shot number predicting function displays the number of images that can still be shot. According to a film camera, since a piece of film is consumed for each shooting, it is possible to clearly know the number of remaining images to be shot. However, according to a digital image pick-up apparatus which saves the photographs as data in a digital format, since it is normal that the portion of a data recording medium consumed by one shot is not fixed, the user cannot clearly know the number of images that can still be shot. If the user cannot know the number of remaining images, it is difficult for the user to form a shooting plan and it is a disadvantage for the user. Therefore, conventionally, in the digital camera that is the specialized photographic machine, a function that makes efforts to maintain a uniform consumption of the data recording by each shooting so as to make it possible to estimate the number of remaining images to be shot is mounted.
In order to easily estimate the number of remaining images, it is effective to make the data size of the image data file to be generated by each shooting uniform because the number of photographable images can still be shot can be calculated from the remaining capacity of the data recording medium if the data size of the image data file generated for each shot is regular. In the meantime, since more image data are saved in the data recording medium, the image data is normally compressed and then saved in the data recording medium. One or a plurality of parameters is involved in this compression and a parameter called a quality factor is a typical one in the compression of a JPEG format. The sizes of the data of images after compression using the same parameter value are different depending on the content of the photographed image. Accordingly, when all the image data are compressed by the same compression parameter, the size of the generated image data file is different for each shooting. Therefore, as described in JP-A-4-233373, in the conventional specialized photographic machine, the data of different images generated by shooting is compressed at different compression parameter values and when it is compressed into a desired data size, the compressed image data is saved so as to always maintain a set data size of the compression image data file to be generated. Accordingly, the parameter value of the compression parameter for use in the compression of the image data is slightly different for each shooting.
The structure of the hardware of the conventional specialized photographic machine and a method of deciding a parameter value of a compression parameter will be described with reference to FIG. 7. FIG. 7 typically shows a hardware structure of the conventional specialized photographic machine. A specialized photographic machine 130 is configured by a lens 132, a solid-state image pickup device 133, an A/D converter 134, a DSP 135 for the image processing, a CPU 136, a temporary storage unit 137, a main storage device 138, a display 139, and a user interface 140 such as a shutter button, and the exchange of a signal among these devices is carried out through a bus 131. If the shutter button of the user interface 140 is pressed down, the CPU 136 issues an order to start shooting, the light passed through the lens 132 is converted into an electric signal by the solid-state image pickup device 133, and this electric signal is digitalized by the A/D converter 134 to be temporally stored in the temporary storage unit 137. Here, all of the signals outputted from all pixels of the solid-state image pickup device are stored in the temporary storage unit 137 as the digital data. Next, the DSP 135 reads the output signal of the solid-state image pickup device 133 that is made into the digital data stored in the temporary storage unit 137 to create the image data for one photographed frame from the data. The image data is in a YUV format. The DSP 135 temporarily saves the created image data for one frame in the temporary storage unit 137 again.
Continuously, the DSP 135 reads the image data for one frame that was created previously from the temporary storage unit 137 and it JPEG-compresses this image data at a predetermined compression parameter value, and measures the data size after the compression. The compression format is normally JPEG. When the data size after compression is not a desired data size, the image data for one frame from the temporary storage unit 137 is read again, the compression parameter is changed, it is compressed again, and the DSP 135 measures the data size after compression. Then, repeating the above-described operation, the DSP 135 saves the image data that is compressed at a compression parameter value that the data size after compression becomes the desired data size in the main storage device 138.
However, trying to execute the above-described conventional method of controlling a parameter value of a compression parameter also in the conventional camera-equipped cellular phone, two disadvantages are generated. This will be described with reference to FIG. 8.
FIG. 8 typically shows a conventional camera-equipped cellular phone. A conventional camera-equipped cellular phone 150 is characterized in that it comprises a host module 152 handling functions such as telephone calling and scheduling and an interface 153 is located among the modules. The reason that a conventional camera-equipped cellular phone 150 in particular comprises two modules is that a general versatility is given to the camera module. If a camera part is made into an independent module, it is possible to combine the same camera module with various cellular phones and PDAs.
Such a camera module is obviously forced to be a compact size and cheap. However, if the conventional method of controlling a parameter value of a compression parameter used in the conventional specialized photographic machine is used, a DSP 135 with a high processing ability and a temporary storage unit 137 must be mounted in the camera module, and this makes the camera module a large size and expensive. Since the temporary storage unit 137 must save the image data configuring one frame, the higher the resolution of the image pickup device is, the more the size and the price of the camera module are increased.
Therefore, if the conventional method of controlling a parameter value of a compression parameter is mounted on a host module 152 without providing a compression function of the image data to a camera module 151, the other disadvantage is generated. In this case, at first, the bus width of an interface 153 should be widened so as to be capable of transferring the image data that is not compressed. In order to make the bus width broad, the number of the signal line for this should be increased and this increases the size and the price of the camera module. Accordingly, it is preferable that the image data is compressed by the camera module 151 and then transferred to the interface 153. In addition, it is preferable that the data processing amount is made as light as possible, because while the host module also has a temporary storage device and a CPU, the host module must perform various processing not done by the specialized photographic machine. Accordingly, a method to decide the parameter value of a compression parameter by exchanging the image data between the temporary storage unit and the CPU many times is not effective.
Thus, due to a problem with respect to the sizes and the prices of the camera module and the interface and the data processing capability, the conventional camera-equipped cellular phone and camera-equipped PDA or the like cannot make the data size of the image data file uniform by controlling the compression parameter value of the image data.